The present invention relates to a toner for developing an electrostatic image in an image forming method such as electrophotography, electrostatic recording or electrostatic printing. The present invention is also directed to a developer containing the above toner, to a container containing the developer, to an image forming apparatus having the container and to an image forming method using the above toner.
In an image forming method such as electrophotography, electrostatic recording or electrostatic printing, an electrostatic latent image formed on an image carrier such as a photoconductor is developed by a toner in a developing step. The toner image thus obtained is then transferred from the photoconductor to a transfer medium such as a transfer paper in a transfer step, and fixed on the paper in a fixing step.
The toner contains a colorant, a binder resin and one or more additives such as a charge controlling agent and a wax and is generally produced by a grinding method or a suspension polymerization method. In the grinding method, a blend of the raw material ingredients of the toner is kneaded with a kneader. The kneaded mixture is then cooled, ground and classified. From the standpoint of economy, the raw materials of the toner should be so selected as to provide an easily pulverizable composition in order to permit the of a general pulverizer. In this case, however, the toner particles thus produced have widely distributed particle sizes. In order to obtain a toner capable forming images having high resolution and gradation, therefore, it is necessary to remove coarse particles such as those having particle diameters of 20 μm or more and fine particles such as those having particle diameters of 5 μm or less. As a result, the yield of the toner is unavoidably low. The grinding method has an additional problem, because it is difficult to disperse the colorant and other additives uniformly in the binder resin. Non-uniform distribution of such ingredients adversely affects the fluidity, developing efficiency, durability and image quality of the toner.
To cope with the problems of the above grinding method, the suspension polymerization method has been proposed and is actually practiced. However, toners obtained by the suspension polymerization method are spherical and are difficult to be removed from surfaces of photoconductors. Thus, residual toners remaining on a surface of a photoconductor after the transfer of a developed toner image from the photoconductor to a transfer medium are apt to be retained thereon even after cleaning with cleaning rollers or brushes. The unremoved toner on the photoconductor will cause background stains of images. Further, the unremoved toner will be transferred to a charging roller which is in rolling contact with the photoconductor and will adversely affect the charging performance thereof.
Japanese Patent No. JP-B-2537503 discloses a non-spherical toner in the form of aggregates of secondary particles each of which is composed of fine primary particles of a resin and fine particles of a colorant. The primary resin particles are prepared by emulsion polymerization. While this toner may permit easier cleaning from a photoconductor surface as compared with a spherical toner, a surfactant used during the formation of the primary particles is apt to remain unremoved from the aggregates and pollute the photoconductor, charging roller, developing roller, etc. Further, a releasing agent such as a wax is confined within the aggregate, the desired resistance to offset is not obtainable. Additionally, since the fine particles constituting the secondary particles are not distributed uniformly in the secondary particles, there is a variation in surface characteristics between respective toner particles formed of the secondary particles, so that it is not possible to form uniform images for a long period of time.
Japanese laid-open patent publication No. JP-A-2000-292978 discloses a toner having a core-shell structure in which core particles, each containing a low molecular weight resin and a colorant, are each surrounded by particles of a high molecular weight resin. The high molecular weight resin particles, which form a shell, are fuse-bonded to the core. The presence of the high molecular weight resin particles on outer surfaces of the toner can improve resistance to offset. This toner, however, has a problem, because the fixation of toner images cannot be carried out at a low temperature.